DESCRIPTION: (Adapted from the Applicants' Abstract.) This proposal seeks to add to our knowledge concerning organ of Corti function by examining its mechanoelectric responses to sound using four distinct, but linked measurement approaches; gross cochlear potentials, ear canal otoacoustic responses, intracellular hair cell receptor potentials, and most importantly, basilar membrane motion. Each of these provides a different viewpoint on the performance of inner and outer hair cells. The detailed motion pattern of the basilar membrane will be studied first, using a new method for this purpose. A new and detailed view of the traveling wave pattern will provide an important empirical base for future theoretical models of the organ of Corti function and establish a point of experimental departure to measure olivocochlear efferent activity. Natural, sound evoked, activation of efferents as well as electrical activation will be studied. Ear canal otoacoustic changes during efferent activation will be directly compared to basilar membrane motion changes. This comparison will reveal some mechanical aspects of neurally controlled outer hair cell performance. The role of the efferent system to protect OHCs from the damaging effects of loud sound will be studied, as will how efferent activity can improve our ability to detect and code sounds in the presence of masking noises. The investigators will also study the physiology of the higher brain stem levels of the descending efferent pathways. Electric current stimulation of areas of the inferior colliculus will be used to activate this system, leading not only to new information on the descending pathways proper but perhaps also on the unknown physiology of the "lateral" efferent fibers which terminate on the peripheral processes of afferent neurons.